The present invention relates to a device for turbocharging an internal combustion engine wherein the compressors of two exhaust gas turbochargers are connected serially to a charge air distributor which is common to all cylinders of the internal combustion engine and wherein the turbines are connected parallel to one another to the exhaust gas manifold of the internal combustion engine. The two exhaust gas turbochargers can be switched sequentially. The compressor of the exhaust gas turbocharger which operates alone when only a minimal charge air volume is needed is arranged between the charge air distributor and the compressor of the second exhaust gas turbocharger that is capable of transporting a larger air volume than the first exhaust gas turbocharger.
By means of turbocharging, small size internal combustion engines can be provided with increased output matching that of a substantially larger internal combustion engine. In the case of a preset output, the engine can be reduced in size when turbocharging is employed. Turbocharging of internal combustion engines with exhaust gas turbochargers, however, entails the problem that, on the one hand, the turbocharging results in an increase of the output, but, on the other hand, the geometric mechanical compression must be reduced proportional to the desired output. The reduction of the geometric mechanical compression, however, results in the output, respectively, the torque of the turbocharged internal combustion engine to drop at lower rpm ranges more than proportionally which is a result of the peak-shaped output curves of turbocharged engines within their own operational rpm diagram. A turbo engine can provide an optimal flow and thus a high output only within a very narrow rpm range. However, this is contrary to what is desired for internal combustion engines of land vehicles for representing a pulling force hyperbola. When employing several exhaust gas turbochargers, it is possible to divide the operational rpm range of the internal combustion engine as well as the specific volumetric working size of the exhaust gas turbochargers. The switching on of exhaust gas turbochargers in a time, sequence, however may cause the turbocharging system to reach the range of pumping which results in the collapse of the charge air flow in the newly switched-on compressor when, for example, the air, flowing from the second compressor with minimal pressure first via a relief valve into the atmosphere, enters the charge air distributor of the internal combustion engine which operates already under full pressure of the first compressor. In this scenario the air has to flow from the turbo engine with relatively great flow velocity and minimal pressure building capability to a piston engine with relatively small flow velocity and relatively high pressure building capability.
A turbocharging device described in German Patent Document 297 23 421 has the unique feature that the compressor of the greater exhaust gas turbocharger has arranged downstream thereof a third compressor of a third exhaust gas turbocharger that is connected serially to the other compressors. The third exhaust gas turbocharger comprises a turbine provided with its own bypass valve for connecting the turbine in parallel to the first turbine to the exhaust gas manifold, wherein the air volume to be conveyed by the third turbocharger is greater than that of the larger one of the two other exhaust gas turbochargers. In this way, the output increase of the internal combustion engine also results in lowered fuel consumption and reduced pollutant emission, improved acceleration behavior at low rpm, and a harmonic transition of the operational spectrum from very low rpm at partial load and full load to high rpm at partial load and full load. Only the smallest one of the compressors conveys charge air constantly in this arrangement of three serially connected compressors. In the two larger compressors, which are positioned remote from the engine, the pressure is essentially increased in order to force the charge air required within higher operational ranges of the internal combustion engine without interruption through the limited cross-section of the already fully loaded smallest compressor adjacent to the engine. The pressure drop, occurring after passing of the charge air through the smallest compressor, to the pressure ratio produced by this first smallest compressor results in a considerable temperature reduction which can be used for an advantageous increase of the geometric mechanical compression in the cylinders of the internal combustion engine. In practice, it is impractical because of the different sizes of the exhaust gas turbochargers to introduce into the serial compressor arrangement an additional compressor of a fourth exhaust gas turbocharger. A finer division of the operational spectrum of the internal combustion engine by a stepped operation of more than three exhaust gas turbochargers without disruptive pumping effect requires further measures.
It is therefore an object of the present invention to lower for a turbocharged internal combustion engine the fuel consumption and the pollutant emission, to improve the acceleration behavior at low rpm ranges, to provide a harmonic transition of the operational spectrum from very low rpm at partial load and full load to higher rpm at partial load and full load in connection with an output increase and to realize a stepped on and off switching of a larger number of exhaust gas turbochargers without disruptive pumping effects.